1. Field of the Invention
The present invention relates to an exhaust gas treating apparatus and a method thereof, and more specifically to an exhaust gas treating apparatus and a treating method for a carbon dioxide capture process, in which harmful substances remaining in the exhaust gas discharged from the conventional flue-gas desulfurization process are additionally removed for efficient performance of the carbon dioxide capture process.
2. Background Art
With the earnest development of CO2 capture and storage (below to be referred to as ‘CCS’) technology for reduction of greenhouse gas emissions, the level of carbon dioxide capture technology has reached the commercialization stage, and CCS equipment is being installed and operated in existing thermal power generation facilities.
In particular, coal discharges a lot of carbon dioxide because its carbon (C)/hydrogen (H) ratio is high among fossil fuels. So a major objective for the coal thermal power plant using coal as fuel is to reduce CO2 emissions.
FIG. 1 is a block diagram showing an exhaust gas treating system of a conventional coal burning thermal power plant.
With reference to FIG. 1, in the conventional exhaust gas treating system of a coal burning thermal power generation plant, the contaminants are removed from the exhaust gas discharged from the boiler 110 through flue-gas denitrification equipment 120, dust-collecting equipment 130 and flue-gas desulfulization equipment 140, before carbon dioxide is captured through carbon dioxide capture equipment 150 to be compressed and stored, and the exhaust gas is discharged into the atmosphere through a stack 160.
As shown in FIG. 1, in the case of the conventional exhaust gas treating system of a coal burning thermal power plant, most of them are equipped with flue-gas desulfurization equipment 140 and flue-gas denitrification equipment 120, so about 90% of sulfur oxide (SOx) and nitrogen oxide (NOx) contained in the exhaust gas is removed before it is discharged.
However, even after the exhaust gas has passed through the denitrification equipment and desulfurization equipment, the exhaust gas contains about 50 ppm to 100 ppm of SOx and NOx. If the exhaust gas containing sulfur oxides (SOx) and nitrogen dioxide (NO2) that exists at about 5% in total nitrogen oxides (NOx) is introduced into a carbon dioxide (CO2) capture process, the operation efficiency and the economic efficiency decrease due to degradation by an absorbent, especially an amine based absorbent or an alkali absorbent, used in the carbon dioxide (CO2) capture process.
Furthermore, the recycling slurry of the flue-gas desulfurization equipment contains about 8,000 to 20,000 ppm of chlorine ions (Cl−) and about 4,000 to 8,000 ppm of sulfate ions. If these ions are contained in fine droplets and finally introduced into the carbon dioxide (CO2) capture process, they react with the absorbent to form sulfate (SO42−)/chloride (Cl−) compounds so as to lower the reactivity of the absorbent. Therefore, inflow of fine droplets containing such ions should be thoroughly cut off.
In the conventional coal-burning thermal power plant, usually a selective catalyst reduction (SCR) process is used for the flue-gas denitrification process to treat nitrogen oxides. At this time, the removal efficiency is determined in accordance with the NOx emission standard, and the change and the charge of catalyst or the feed of ammonia or urea solution is determined accordingly. At this time, in order to prevent the inflow of nitrogen dioxide (NO2) existing at about 5% in nitrogen oxides into the carbon dioxide (CO2) capture process, the additional apparatuses are necessary.
Coal contains harmful heavy metal components (HAPs) such as mercury, and they are finally discharged into the atmosphere in the combustion process. So various methods, such as an oxidation catalytic treatment apparatus, a halogenation treatment apparatus or an activated carbon adsorption tower, have been developed to remove HAPs from flue gases.
In order to apply a CO2 capture process to a coal burning thermal power plant wherein exhaust gases containing various contaminants are discharged, a pretreatment process is required to maintain sulfur dioxide (SO2) below 10 ppm and remove contaminants. Accordingly, there has been an increasing necessity that the existing flue-gas desulfurization process is substantially retrofitted or the secondary flue-gas desulfurization equipment is additionally installed.
However, to maintain the concentration of discharged sulfur dioxide (SO2) below 10 ppm by raising the desulfurization efficiency to about 99.5% using the existing flue-gas desulfurization process, the physical size of the absorption tower should be significantly increased and the packing materials and interior structure should be completely retrofitted.
Also, to increase the circulation rate of the absorbent slurry, basic equipment or apparatus such as a circulating pump should be added or replaced with the larger one. Therefore, in the case that CO2 capture equipment is to be added to the equipment that was already installed and is in operation, it is impossible to use the existing flue-gas desulfurization equipment by retrofitting it and additional secondary flue-gas desulfurization equipment should be introduced.
FIG. 2 is a block diagram showing an exhaust gas treating system provided with flue-gas desulfurization equipment of a conventional coal burning thermal power plant.
As shown in FIG. 2, in a conventional exhaust gas treating system 200 of a coal burning power plant, after removing contaminants from the exhausts gas discharged from a boiler 210 through flue-gas denitrification equipment 220, dust collecting equipment 230 and first flue-gas desulfurization equipment 240 and secondary flue-gas desulfurization equipment 250, carbon dioxide is captured through carbon dioxide capture equipment 260, and it is compressed and stored, and the exhaust gas is discharged into the atmosphere through a stack 270.
On the other hand, the secondary flue-gas desulfurization equipment 250 is equipped with a mercury halogenation apparatus 251, a secondary desulfurization and mercury absorption apparatus 252, an ion absorption apparatus 253 and denitrification and absorption equipment 254, and these are connected in series.
Since the secondary desulfurization equipment has various parts arranged in series to remove contaminants contained in the gas discharged from the first desulfurization equipment, a large installation space is necessary. Also, an enormous installation cost is required and the operation cost for the processes also rapidly increases, so there is a problem of overall cost increase.